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The lecture video is embedded below but also available here in MP4 format.
Additionally, slides used in the lecture are embedded below but also are available here in Powerpoint format.
Questions after the lecture? Please ask them in here.

Additional Apod photos disccussed

Imagine if we lived in a binary solar system or a triple star or even an open cluster with 10-100 stars or why not a globular cluster with 100000-1000000 stars….. But don’t we see that we live with one bright star, which is kind a bit unusual, but it rather appears that it is the greatest benefit to humanity, allowing us to have a Night.

The winter sky is an excellent place to begin exploring the constellations that make up the night sky. Orion is the key, or signpost, for locating many of the other constellations in the winter sky. There are two convenient ways to locate all of the main constellations around Orion once Orion is located. Fortunately, Orion is easy to locate and well known to most people.

The first way is to follow lines made by pairs of stars in Orion. The second way is to locate the great winter hexagon of bright star around Orion.

The Constellations of the Winter Sky

If you live in the northern latitudes and you scan the sky from the southern horizon to the region overhead, you should be able to see the following constellations on a clear winter night: Orion the Hunter, Canis Major the Great Dog, Canis Minor the Little Dog, Taurus the Bull, Auriga the Charioteer, Gemini the Twins and the Pleiades star cluster. (See the map on the next page).

In Greek mythology, Orion was a great hunter who eventually offended the gods, especially Apollo. Apollo tricked Artemis, the Goddess of the hunt, into shooting Orion on a bet. When she discovered that she had shot Orion, she quickly lifted him to the heavens and made him immortal, where he now hunts eternally with his two dogs, Canis Major and Canis Minor. In front of him is his prey Taurus the Bull.

The myths surrounding Auriga the Charioteer vary, but it is an ancient constellation dating back to at least to the Ancient Greeks. Some say Auriga invented the chariot and others that he trained horses for the best chariots.

Gemini is a constellation made up of two stick figures known as the twins, Castor, who was a great horseman, and Pollux, who was a great boxer. According to one myth, Castor and Pollux (a.k.a. Polydeuces) were the sons of Zeus and Leda (from Leda and the Swan) and were hatched from an egg. Their sister was the beautiful Helen whose face launched a thousand ships to do battle in front the Trojan city of Troy.

Method 1: Using Pairs of Stars in Orion as a Guide

Finding Sirius and Canis Major

If you follow a line from the belt stars of Orion to the left and slightly down, you will come across a very bright star called Sirius, which is also known as the Dog Star. (See the arrows in the diagram to the right).

Sirius is the brightest star in the night sky so it is hard to miss. Once you’ve located Sirius you can locate the other stars in the constellation Canis Major the Great Dog.

Finding Procyon and Canis Minor

Follow the a line from the shoulder stars of Orion to the left. The first bright star that you will come close to is Procyon, which resides in Canis Minor.

From there you should be able to see the other star that us easily visible. Together, the two stars make up the constellation Canis Minor, which is also known as the Little Dog. Along with Canis Major, Canis Minor follows Orion across the heavens on an eternal hunt.

Finding Aldebaran and Taurus

Following the belt stars to the right, you will pass just below the bright star Aldebaran and through the constellation Taurus, which is also known as the Bull.

Continuing on you will run across a fuzzy blur of stars closely grouped. These are the Pleiades, or the Seven Sisters.

Finding Capella and Auriga

Follow the bottom most star on the left and the left most belt star upwards (going roughly over your head) and you will come across a very bright star called Capella. From Capella, you can follow the pentagon of brighter stars nearby that make up Auriga. Just below Capella, there is a triangle of stars known as ‘the kids’ as in goat babies.

Capella was one of the most important stars for navigation as it could be seen throughout most of the year from mid northern latitudes.

Finding the Twins Castor and Pollux

Follow a line from Rigel to Betelgeuse heading upwards and overhead. You will come to two rough sticks of stars that are headed by two brighter stars. This is the constellation Gemini, composed of the twins Pollux and Castor. Pollux is on the left and Castor is on the right.

Method 1: Using the Winter Hexagon Centered About Orion

If you look in around the sky centered on Orion, you should be able to see a rough hexagon of very bright stars. This is called the Winter Hexagon. Starting at Rigel, if you go counterclockwise by one, you end up at Aldebaran in Taurus. Go counterclockwise once more and you end up at Capella in Auriga. Go counterclockwise once more and you end up at the pair of stars Pollux and Castor in Gemini. Go counterclockwise once more and you end up at Procyon in Canis Minor. Finally, if you go counterclockwise once more you end up at Sirius in Canis Major.

The winter hexagon is not a constellation but simply an asterism. But when there’s winter in the Northern Hemisphere, its summer in the Southern Hemisphere. I find it really unfair for the world to call this set of stars as the winter hexagon. Why don’t call it the summer hexagon? Frankly speaking I feel like being discriminated on belonging from the Southern Hemisphere.

Winter Hexagon from the Tropics

I understand that development in the astronomy field knew its leap in the Northern Hemisphere, but still I am hereby campaigning for a change in the name of this asterism ( I know Asterisms are not even officially recognized names). But the “winter hexagon” It is a complete misnomer. When I first got to see this beautiful set of constellations in its entirety by the seaside, I was feeling hot. One as it was a breezeless night, hot and damp. Secondly for the utter pleasure of being able to identify the hexagon which covered almost ¼ the portion of the sky and being able to identify six constellations in one go. It was awesome.

One could ask it’s already April and the spring is already here, so why talk about the winter hexagon? It’s just because the winter hexagon in a few months would not remain in our skies. During January at dusk I would have to raise up my head towards the zenith to see the Orion as it would highlight our north western skies, but now in April it’s already halfway between the horizon and the zenith towards the west at dusk. And to tell you Scorpion is already on its way. (Hope you know about the Scorpion – Orion saga) So to say in a few months Orion would be no more on the skies. (I would miss Orion a lot)

And as April is here, it would be a lovely time to appreciate the nature in its bloom and a have good time to observe the night sky. No more shivering and complaining about the chilly weather to have a look at the sky (for my friends of the Northern Hemisphere). As for us Mauritians, we have only two seasons per se. Our hot humid summer is already gone and we are slowly entering the winter phase.

So, if you feel being discriminated by pronouncing the Winter Hexagon, (or any other misnamed constellation or asterism) then campaign with me for this misnomer. Together, united we can change the name of this marvelous hexagon to a common name. Because as the saying goes “the sky has no borders, it is for everyone”. (Is it really a saying or I just made it up?). I suppose I got it from The Astronomers without borders. They have their motto as One people, One Sky.

This is a list of the 20 brightest stars as seen from the Earth (not including the Sun). The stars are numbered from 1 to 20 in sequence.

Common Name

This is the name by which the star is commonly known. The names are Greek, Latin or Arabic. This web site is based in London: stars not visible from London are in red.

Some examples of the names: Deneb is Latin for tail(because it marks the tail of The Swan – Cygnus); Antares is Greek for rival of Mars(because of its red colour); Aldebaran is Arabic for eye of the bull (because it marks the eye of The Bull – Taurus).

Constellation

A constellation is a star group (as seen from Earth) that the star is a part of. Constellations are human inventions. The stars in them appear in the same part of the sky but are, in fact, at different distances from us and not related to each other. Different cultures use different constellations. For more, read Astronomy and Astrology.

In the West, there are 88 recognised constellations; 48 of these date from Roman times and are known as the Classical Constellations. These include the 12 Zodiac constellations through which the Sun, Moon and planets always pass through. Constellations are always known by their Latin names.

Constellations are used by astronomers for convenience. We say that Sirius is in Canis Majorrather than give its celestial coordinates.

Apparent Magnitude

Apparent Magnitude tells how bright the star is as seen from the Earth. The magnitude scale was devised by the Ancient Greeks. The brightest stars were called First Magnitude, the next brightest were called Second Magnitude, etc.

In modern times, the scale has been defined mathematically. A star of magnitude 1 is about 2.5 times brighter than a star of magnitude 2 which in turn is 2.5 times brighter than a star of magnitude 3. The brighter a star, the smaller its magnitude. Many stars are brighter than first magnitude. Some stars are so bright they have negative magnitudes. On this scale, Jupiter has a magnitude (at its brightest) of -2.6, Venus is at -4.4 and the Sun -27. The faintest stars visible to the naked eye are sixth magnitude. Pluto has a magnitude of +14, far too faint to be visible without a powerful telescope.

In the table it can be seen that Betelgeux varies its magnitude – some stars are variable in brightness.

The brightness of a star as seen from Earth depends on its intrinsic luminosity and its distance from Earth. A dim star may appear bright because it is close while a luminous star may appear faint because it is far away. This is why we sayApparent Magnitude.

Spectral Type

When starlight is passed through a prism, it splits into its constituent colours, like a rainbow. This is called the star’s Spectrum. Stellar spectra are crossed by dark lines. These lines give astronomers a lot of information about the star: temperature, luminosity, radius, magnetic properties, movement. Read The Electromagnetic Spectrum for more on spectra.

The Morgan-Keenan spectral classification (Photo credit: Wikipedia)

Stellar spectra are classified into types. These types are given letters. The spectral type series is a temperature series. Moving from the hottest stars to the coolest, the series of letters runs O, B, A, F, G, K, M.

Each spectral type is subdivided into ten numbers. For example, A0, A1, A2, up to A9. A0 is hotter than A1. The table below gives more information.

Spectral
Type

Colour

Surface
Temperature
(°C)

O

Blue

>30,000

B

Blue-White

20,000

A

White

10,000

F

Yellow-White

7,000

G

Yellow

6,000

K

Orange

4,500

M

Red

3,000

Our Sun is a star of Spectral Type G2 with a surface temperature of around 6,000°C.

Luminosity

This tells us how much more energy and light the star gives off compared with the Sun. This is how bright the star really is once distance has been taken into account. There is a huge variety in the luminosity of the stars. At one extreme, the star Alpha Centauri is 1.7 times more luminous than the Sun. At the other extreme, Canopus is 15,000 times more luminous than our Sun.

Luminosity can be measured indirectly by combining the apparent brightness of a star with its distance. It can also sometimes be measured directly from the spectrum.

Distance

The distance of a star is given in Light Years. This is the distance covered by a light beam in one year. Light travels at 300,000 km per second (186,000 miles per second). In one year a beam of light will travel 9.4 million million km (5.9 million million miles). This enormous distance is a Light Year.

Many stellar distances can be measured directly by trigonometry. As the Earth moves around the Sun, the star appears to shift its position against more distant stars. This effect is called parallax. It is a tiny effect but can be measured. The amount of the parallax depends on the diameter of the Earth’s orbit around the Sun (just under 300 million km or 186 million miles) and the distance to the star. A star with a paralax of 1 second of arc (written 1″) is said to be at a distace of 1 Parsec. 1 Parsec is equal to 3.26 Light Years.

Other stars can have their luminosity measured by their spectra or by other properties. When this is compared to their apparent brightness, a distance can be calculated.

This the velocity of the star relative to the Sun. Negative velocities denote a star moving towards the Solar System. Positive velocities are for stars moving away from us.

Radial velocity is easily measured by looking at the star’s spectrum. The lines on the spectrum are shifted to the blue end if the star is moving towards us (the so-called blue shift) and to the red end if the star is moving away from us (red shift). The amount of this shift depends on the relative velocity between us and the star.

The following list contains the 100 brightest stars as seen from the earth at night. The information on magnitudes is taken from data obtained by the Hipparcos Satellite Catalog. Distance measurements are from the Observer’s Handbook 2001, by The Royal Astronomical Society of Canada.

* Bayer designation: names given to stars by astronomer Johanne Bayer in his 1603 star atlas Uranometria. The designations consist of a Greek letter followed by the genitive (possessive) form of the constellation name that the star is found in. They were generally named starting with the brightest star and continuing to the dimmest of any given constellation.

A lecture to study the nerdy part of the Big Universe. Do you think it’s a good way to kick-start your Sunday morning? It’s what I did. It was a mind-blowing experience. But what intrigues me is: Why is this Universe so big?

The lecture video is embedded below but also available here in MP4 format.
Additionally, slides used in the lecture are embedded below but also are available here in Powerpoint format.
Questions after the lecture? Please ask them in here.